The recent trend of mining and processing considerable volumes of finely disseminated ores has brought about an increased demand for screening processes and corresponding plant capable of intensifying the operations of concentration and extraction of finely granular useful minerals. Thus, there has been created a situation of the growing significance and topicality of development work aimed at creating methods and devices stepping up the technological ratings of the screening process, such as its efficiency understood as the capacity of extracting fine size fractions from an initial solid material at high specific throughput and productivity.
There is known a method of hydraulic screening of solid materials (U.S. Pat. No. 3454162) including the steps of feeding a solid material as a slurry onto the sifting surfaces, oscillating this surface and supplying thereupon an auxiliary fluid.
There is also known an apparatus for hydraulic screening of solid materials (U.S. Pat. No. 3454162), comprising a box having mounted thereon a vibrator and a frame with the sieve secured thereto, a slurry feed pipe and pipes for supplying the auxiliary fluid.
However, these known method and apparatus are characterized by relatively low efficiency and specific throughput of the screening operation, particularly in classification of finely granular materials, which is caused, among other things, by the effects of surface tension of the fluid and adhesion among particles. This tends to form films on the sifting surface, closing its openings, whereas the material itself tends to form lumps or clods obstructing the progress of fine particles towards the sifting surface and through its openings, so that the process ratings of the screening operation are adversely affected. Although the supplying of the auxiliary fluid prompts the breaking of these lumps or clods, this takes place only in the zones of interaction of the material with the jets of the auxiliary fluid, and with the material passing these zones relatively quickly, is of but little practical effect.
There is further known a method of screening solid materials (U.S. Pat. No. 975115), including the steps of feeding a solid material as a slurry onto the sifting surface of the screening apparatus, set at an inclination angle in the direction opposite to that of the progress of the solid phase of the slurry, oscillating this sifting surface, and supplying an auxiliary fluid thereupon.
There is also known an apparatus for screening solid materials (U.S. Pat. No. 975115), comprising a box having mounted thereon a vibrator and a frame with a sieve secured thereto, the loading end of the box being situated below the discharge end thereof and submerged in a receptacle with a liquid, and pipes feeding the slurry and the auxiliary fluid, overlying the sieve.
In this known apparatus the vibration of the box and of the sieve connected therewith excites oscillations of the slurry and of the liquid, yielding a fluidization effect. Although lessening the tendency of lump or cold formation on the sieve surface, this effect, however, opposes the passage of fine particles through the mesh of the sieve. The auxiliary fluid supplied onto the liquid-air interface zone of the sieve promotes the passage of the smaller size fraction of particles within this zone, but on the whole, the known method and apparatus fail to offer sufficiently high process ratings, as the efficient separation of the initial material into size fractions actually takes place only over the limited area of the liquid-air interface.
There is still further known a method of screening solid materials (U.S. Pat. No. 975117), including the steps of feeding a solid material onto the sifting surface of the screening apparatus and oscillating this surface by an effort applied at an acute angle to the longitudinal axis of the sifting surface, causing the motion of solid material particles in a direction normal to the sifting surface, and from the loading zone towards the discharge zone.
There is also known an apparatus for screening solid materials (U.S. Pat. No. 975117), comprising a box mounted on resilient suspension members, carrying thereon a vibrator and a support frame having a sieve secured thereto, associated with a pipe for feeding the solid material.
The last-mentioned known screening method and the apparatus capable of performing this method are more efficient than the previously discussed methods and apparatuses of the prior art, also in the screening of finely granular solid materials fed in the form of a slurry. This is due to the cyclic motion of the material over the sifting surface in both longitudinal and transverse directions, attained by reversing periodically the rotation of the unbalanced weights of the actuating vibrator. This manner of the motion of the solid material prompts disintegration of lumps or clods, enhancing the interaction of particles of the material with the sifting surface. However, the reversing of the rotation of the unbalanced weights of the vibrator results in the material moving in the longitudinal and transverse directions in its progress from the loading zone towards the discharge zone in discrete steps. By stepping up the time of interaction of the material with the sifting surface, this essentially upgrades the screening efficiency that can be as high as 97-99%; however, the specific throughput is cut by some 40%.
With this known method, the motion of the material particles over closed paths can take place only within a relatively thick layer of the material having an exposed surface. In case of a thin layer, the circulation effect is practically non-existent, which lowers the screening efficiency.
The operation of the known apparatus performing the last-described method of screening solid materials requires either the presence of an operator or the incorporation of an intricate automatic control system.